In many image projection systems an optical wavelength conversion material is used as a light source. The wavelength conversion material is excited by an excitation light, such as a laser, and in response the conversion material emits light at a wavelength different from the wavelength of the excitation light. Many wavelength conversion materials such as phosphors and quantum dots (QDs) are thermally limited. The high energy density at the excitation light spot is typically reduced by rotating the conversion material in order to move the excitation light spot and thereby distribute the heat of excitation throughout a larger volume of the conversion material. The conversion material can also be placed on a rotating solid disk which can act as a heat sink and can absorb and dissipate some of the excitation heat from the conversion material.
In order to further increase the rate of heat removal and dissipation from the conversion material, various solutions have been proposed. One approach is to use as the heat sink a larger diameter, solid disk rotating at a higher speed. However, such a larger and heavier rotating disk can be hard to manage mechanically and difficult to package in a projection system. Even if multiple parallel fins are added to the solid rotating disk, the air flow and thermal conductivity constraints can limit the usefulness of the fins: if the fins are spaced too closely, the air between them can stagnate, thereby hindering heat dissipation from the fins. If the fins are spaced too far from one another, there can be a large temperature drop between the base of the fins closer to the conversion material and the bases of successively further fins. The further the fins are from the conversion material, and the lower their base temperature, the lower the rate at which they can dissipate heat. As such, using multiple parallel fins can provide only diminishing returns in the form of increased heat dissipation rate.
Another class of approaches uses liquid cooling to increase heat dissipation rate. For example, the conversion material and/or the heat sink can be partially or fully submerged and rotated in a bath of the cooling liquid. This approach can present challenges rated to liquid containment, compatibility of the cooling liquid with the conversion material, and optical aberrations caused by the system and in particular by the cooling liquid. Many of these liquid-cooled approaches also require rotating mechanical fluid seals, which have a limited lifetime and can require frequent inspection, maintenance, and replacement.